1. Field of the Invention
The present invention relates to a fault diagnostic apparatus and, more particularly, to a fault diagnostic apparatus for diagnosing a fault of a temperature sensor such as a water temperature sensor.
2. Description of the Relevant Art
Water and oil are useful for cooling an engine. The temperature of water or oil is one item of data for judging a state of the engine. For example, under idle speed control, an air flow rate is determined depending on the water temperature, and a corrective factor for correcting a basic fuel injection amount for the sake of improving a warm-up characteristic and an accelerating ability and cleaning emission gas is determined depending on the water temperature.
When this water temperature sensor for detecting a water temperature has a fault, control of each kind is carried out based on an abnormal output signal, and therefore, there is a possibility that some problems may be caused. In order to solve this problem, techniques of several kinds for diagnosing a fault of a water temperature sensor have been proposed.
For example, Japanese Patent Application Laid-Open No. 11-82144 discloses an invention wherein a water temperature sensor is judged as being in an abnormal condition when a cooling water temperature detected after an engine was judged as being warmed up is lower than a predetermined water temperature. Japanese Patent Application Laid-Open No. 2004-76689 discloses an invention wherein by estimating a cooling water temperature based on a parameter related to a heating value of an engine (an amount of heat to be transferred from the engine to the cooling water) and a parameter related to a quantity of radiant heat of the cooling water, and by comparing the estimated cooling water temperature with a cooling water temperature detected by a water temperature sensor, whether a flow regulating valve for regulating a flow rate of the cooling water is abnormal or not is checked.
However, in both of the inventions described in the Patent Documents 1 and 2, a fault of the water temperature sensor or the flow regulating valve is diagnosed with the engine running (i.e. in a situation where the temperature of the engine to be a chief source of heating the cooling water is high). When the temperature of the engine is high, the heating value from the engine also becomes high, leading to a rise in cooling water temperature. As a result, since the cooling water temperature varies widely under a great influence of the heating value from the engine, there is a possibility that a very high degree of estimate accuracy of the cooling water temperature cannot be expected while the engine is running.